Effects of nutrient enrichment on alkaline phosphatase activity and nitrogen fixation potential in stream periphyton.

In an initial experiment, I evaluated methodological considerations of using a novel nutrient-diffusing substrata (NDS) design to estimate the effect of nitrogen (N) and phosphorus (P) enrichment on alkaline phosphatase activity (APA) and N2 fixation. Diffusion rates were effectively estimated, and suggested that this NDS design may also be useful for long-term studies. Levels of APA and N2 fixation potential were substantial in controls, indicative of differential nutrient limitation in the periphyton. In a second experiment, I investigated the effect of ambient N and P concentrations on periphyton accrual, APA, and N2 fixation using a P gradient of selected stream sites, which incidentally served also as an N gradient. Presence of both APA and N2 fixation on controls, which decreased up the P gradient, suggested that periphyton communities were differentially limited by both N and P. This was further corroborated by responses to NDS enrichments of N and P.